Quantum Key Distribution and Qubits ECES Practice Questions (Page 7)

Questions 31–35

1. 31application · medium

   A company is planning to deploy QKD between two buildings that are 10 km apart in a dense urban area. They are considering using free-space QKD. What is a major practical challenge they will face?

   • AThe high cost of optical fiber, which is more expensive than free-space equipment
   • BThe requirement for a vacuum between the transmitter and receiver
   • CThe inability to operate at night due to lack of sunlight
   • DThe need for a clear line of sight, which may be obstructed by buildings or other structures
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2. 32expert · hard

   A telecommunications company is planning to deploy QKD across a metropolitan area network. They need to cover a distance of 150 km, but current QKD systems have a maximum range of about 100 km without repeaters. Which approach should they take?

   • AUse free-space optical QKD to cover the distance without repeaters.
   • BDeploy trusted relay nodes at intermediate points to forward the key.
   • CUse quantum repeaters, which are fully developed and commercially available.
   • DIncrease the power of the laser to extend the range of the QKD system.
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4. 33expert · hard

   An eavesdropper (Eve) intercepts photons in a BB84 QKD session and measures them in a random basis, then forwards them to Bob. What is the most likely effect on the QKD session?

   • AEve's interception will only affect the bits where her basis matches Alice's, leaving the rest unchanged
   • BEve's measurements will introduce errors in Bob's results, which Alice and Bob will detect during the error rate check
   • CEve will be able to perfectly copy the photons and forward them, so no errors are introduced
   • DEve's interception will cause the QKD session to fail immediately, preventing any key from being generated
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5. 34foundation · easy

   What is a major practical limitation of current fiber-based QKD systems?

   • AThey cannot be used with existing telecommunication infrastructure.
   • BThey require a classical channel that is slower than the quantum channel.
   • CThey are vulnerable to quantum computers that can break the key exchange.
   • DThey can only operate over short distances due to photon loss in optical fibers.
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6. 35foundation · easy

   What is the primary purpose of Quantum Key Distribution (QKD)?

   • ATo provide a public-key infrastructure based on quantum entanglement.
   • BTo encrypt data directly using quantum states instead of classical algorithms.
   • CTo increase the speed of classical encryption algorithms.
   • DTo securely exchange a symmetric encryption key between two parties with detection of any eavesdropping.
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